This invention relates generally to means for preventing rotation of steam turbine wheels relative to each other in the event the interference shrink-fit between a wheel and the steam turbine shaft loosens, and particularly, to a captured radial key disposed in a keyway between each adjacent turbine wheel.
Some steam turbines utilize such large rotors that the turbine wheels, which carry the steam turbine blades at their radially outer portions, are not an integral part of the shaft of the rotor. The radial dimensions of such steam turbine rotors may be on the order of seven or eight feet excluding turbine blade dimensions. It is well known in the art that such large rotors are subjected to substantial stresses due to their size, and to quality and quantity of steam affecting steam turbine blades.
Each wheel includes a hub section generally at a radially inner portion of the wheel and each hub section includes a bore therethrough. A wheel, which is not an integral portion of the shaft, may be secured to the shaft by an interference shrink-fit between the radially inner surface of the hub defining the wheel bore and corresponding surface of the shaft. During normal and expected turbine operations, this interference shrink-fit prevents rotation of the wheel relative to the shaft and relative to other wheels secured to the shaft.
To ensure proper operation of the turbine, it is required that turbine wheels be maintained at a substantially fixed circumferential position relative to the shaft and at a fixed axial position relative to other wheels on the shaft. These requirements must be met during all turbine operating conditions, including normal but non-steady-state conditions, such as overspeed while setting the limits, and during undesirable abnormal conditions, such as overspeed due to control malfunction or runaway, and during thermal transient periods.
It is recognized by persons of ordinary skill in this art that certain portions of a wheel, such as radially inner portions, referred to generally as the hub of the wheel, are under substantial stress due to the interference shrink-fit of the wheel on the shaft. This stress, in combination with other stresses generated by normal operation of the turbine, operation under transient thermal conditions or unavoidable adverse operational conditions, has been suspected of causing stress-corrosion cracking indication in portions of the hub of wheels. The precise mechanism which produces stress corrosion cracking is not fully understood; however, it is believed that if stresses at the wheel bore, and generally at the surfaces of the hub are kept at a minimum, and accumulation of water condensed from steam, along with the concentration of oxygen in condensed water and/or steam, is minimized and/or eliminated in the region of the hub, then the probability of stress corrosion cracking occurring will be reduced if not eliminated.
U.S. Pat. No. 4,029,437, by Aubry et al., discloses a cylindrical button disposed in a keyway formed by adjacent hubs of adjacent wheels. The button and keyway are intermediate the periphery of the hub and radially inner portions of the hub adjacent the shaft surface. However, studies have shown that the precise shape of the keyway, formed by opposing axial end faces of the hubs of adjacent wheels, is a critical factor in reducing local stress in that region. Known devices do not address the precise geometric configuration of the key, keyway, and related portions of the hub.